← WA Computer Science ATAR Unit 1

Length check with len()

A length check compares len() of the text with a required length or limit, such as exactly 4 characters for a PIN, to accept or reject the input.

Part of: WA Computer Science ATAR Year 11 — Control Structures and Data Types — Module 93

Understanding Length check with len() in WA Computer Science ATAR Unit 1

What it does: A maximum length check uses <= with len() so that 12 or fewer characters pass. Using != with len() catches any PIN that is too short or too long, so the program can reject it. len(pin) gives the number of characters in the text, so len(pin) == 4 tests for exactly 4 characters. Comparing pin itself to 4 compares text with a number. Use == to compare. A single = is assignment and causes a syntax error inside an if statement. len('12345') is 5, which is not equal to 4. The check does not care what the digits are, only how many there are.

Common mistakes: A single = assigns a value, so it cannot compare the length. A length check needs the comparison operator ==, as in if len(pin) == 4:. pin is text, so it is never equal to the number 4. The check must compare the length of the text: if len(pin) == 4:.

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Learning

Duo

Which condition checks that a PIN is exactly 4 characters long?

len(pin) == 4pin == 4

Matches

if pin == 4:

↓

The PIN text is compared directly with the number 4. It should compare its length: len(pin) == 4.

Applying

Meanings

Term

len(pin) == 4

Meaning

A length check compares len() of the text with a required length or limit, such as exactly 4 characters for a PIN, to accept or reject the input.

Practice

pin = "4821"

Testing

Gaps

if (pin) == 4:

Check Answer

Spots

1if len(pin) = 4:

2 print('PIN accepted')

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Quick Questions

1. Which condition checks that a PIN is exactly 4 characters long?

A. pin == 4

B. len(pin) == 4 ✓

len(pin) gives the number of characters in the text, so len(pin) == 4 tests for exactly 4 characters. Comparing pin itself to 4 compares text with a number.

2. A password must have a minimum length of 8 characters. Which check accepts it?

A. len(password) >= 8 ✓

B. len(password) <= 8

A minimum length means 8 or more, so use >=. Using <= would only accept passwords of 8 characters or fewer.

3. A username can be at most 12 characters. Which check accepts it?

A. len(username) >= 12

B. len(username) <= 12 ✓

A maximum length means 12 or fewer characters, so use <=.

4. A length check tests the...

A. number of characters in the input ✓

B. meaning of the input

A length check only counts characters with len(). It cannot tell whether the data makes sense, only whether it is the right size.

5. Which line is a correct length check in Python?

A. if len(code) == 6: ✓

B. if len(code) = 6:

Use == to compare. A single = is assignment and causes a syntax error inside an if statement.

Match the Pairs

len(pin) == 4 → Checks that the PIN is exactly 4 characters long

len(password) >= 8 → Checks that the password is at least 8 characters long

Accept a username only if it has no more than 12 characters (maximum length) → if len(username) <= 12:

Reject a PIN that is not exactly 4 characters long → if len(pin) != 4:

if pin == 4: → The PIN text is compared directly with the number 4. It should compare its length: len(pin) == 4.

if len(password) > 8: → Using > means an 8-character password is rejected. For a minimum of 8 it should be len(password) >= 8.

Key Term

len(pin) == 4

A length check compares len() of the text with a required length or limit, such as exactly 4 characters for a PIN, to accept or reject the input.

Fill in the Blanks

1. if len(pin) == 4:

Hint: Type the built-in function that counts the characters in a text value.

2. if len(pin) == 4:

Hint: Type the comparison operator that tests for an exact match in length.

3. if len(password) >= 8:

Hint: Type the operator that means the length must be at least the minimum.

4. valid = len(username) <= 12

Hint: Type the operator that allows a length up to and including the maximum.

5. pin = input("Enter 4-digit PIN: ") if len(pin) == 4: print("PIN accepted")

Hint: Type the required length of the PIN, as shown in the prompt.

Spot the Error

Question 1

A if len(pin) = 4:

B print('PIN accepted')

C else:

D print('PIN rejected')

Line A — A single = assigns a value, so it cannot compare the length. A length check needs the comparison operator ==, as in if len(pin) == 4:.

Question 2

A pin = input('PIN: ')

B valid = False

C if pin == 4:

D valid = True

E print(valid)

Line C — pin is text, so it is never equal to the number 4. The check must compare the length of the text: if len(pin) == 4:.

Question 3

A pw = input('Password: ')

B if len(pw) <= 8:

C print('Strong enough')

D else:

E print('Too short')

Line B — A minimum length check must accept passwords that are 8 or more characters. <= accepts only short ones, so it should be >=.

Question 4

A name = input('Username: ')

B limit = 12

C if len[name] <= limit:

D print('OK')

E else:

F print('Too long')

Line C — len is a function, so it is called with round brackets, not square brackets. It should be len(name).

Question 5

A pin = input('Enter PIN: ')

B while len(pin) != 4:

C print('Must be 4 digits')

D pin == input('Try again: ')

E print('PIN saved')

Line D — A double == compares values instead of storing the new input, so pin never changes and a wrong PIN loops forever. It should be a single = to assign: pin = input('Try again: ').

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